SAW devices have been used as sensors in liquid and gaseous environments. U.S. Pat. No. 4,562,371 discloses a SAW device comprising a ZnO piezo layer on a cut crystalline silicon substrate that propagates Rayleigh waves.
The surface acoustic waves propagate in 3 directions and can be classified as longitudinal wave motion, Normal waves or shear horizontal waves. A class of shear horizontal [SH] waves are called Love waves which are propagated in layered devices that concentrate the wave energy in a highly confined region near to the surface.
Rayleigh wave sensors have been useful in gaseous environments but they are not suitable for liquid environments because the surface-normal displacement causes strong radiative loss into the liquid. For sensing in liquids shear horizontal [SH] polarised wave modes are preferred since the particle displacement is parallel to the device surface and normal to the direction of propagation. This allows a wave to propagate in contact with a liquid without coupling excessive acoustic energy into the liquid. However the SH wave is distributed through the substrate and therefore does not have the same sensitivity as the SAW. For increased sensitivity Love waves which are SH-polarised guided surface waves may be used. The waves propagate in a layered structure consisting of a piezoelectric substrate and a guiding layer which couples the elastic waves generated in the substrate to the near surface. They are extremely sensitive to surface perturbations due to the energy confinement to the near surface. By observing the magnitude of perturbations it is possible to measure the strength of the interaction. The interactions may be caused by mass density, elastic stiffness, liquid viscosity, electric and dielectric properties. The more sensitive is the device the smaller the quantities that can be measured.
U.S. Pat. Nos. 5,130,257, 5,216,312, 5,283,037 and 5,321,331 disclose love mode SAW sensors used in liquid environments. The love waves are produced by cutting the piezo electric material such as lithium niobate, lithium tantalate or quartz to couple energy from the interdigital transducers [IDT's] of the SAW device into shear transverse or love waves that enable the wave energy to be trapped at the substrate surface.
U.S. Pat. No. 5,705,399 discloses a SAW sensor for liquid environments having an AT cut quartz piezo substrate with electrodes connected to a first side in contact with a liquid and a second side that is not in contact. The sensor may be used to detect biological species such as antigens.
U.S. Pat. No. 5,364,797 discloses the use of a porous material as a surface layer of high surface area for SAW devices.
Porous surfaces are used in gaseous environments to increase the contact surface area and consequently the sensitivity of the device. Porous surface have not been used in liquid media because the porous surface increases viscosity which leads to insertion losses and decreasing sensitvity.
It is an object of this invention to improve the sensitivity of SAW sensors particularly in liqid media.